Field of the Invention
The present invention relates to a device for lifting the pole of a tractor-hauled trailer of the semi-carried type, usually referred to as "semi-trailer," this device being also designed for operating as a coupling gear between the trailer and the tractor.
Although the coupling of a trailer comprising two axles (or two groups of axles) on which the load is distributed is not attended by specific difficulties, since the proper relative positioning of the two vehicles is obtainable in most instances by moving one or the other vehicle to the proper coupling relationship, lifting the pole pivoted to the underframe of the trailer requiring only the lifting of the trailer's weight, it is known that coupling a vehicle of the semi-carried type, i.e., wherein the pole rigidly secured to the underframe of the trailer carries a considerable fraction of the transported load, constitutes an entirely different problem.
In the case of a trailer having two carrier axles, the coupling gear secured to the rear of the trailer comprises a hydraulic cylinder and piston unit adapted to move a lever from a low, ground-level picking position to a high, coupling and hauling position, said lever comprising at its rear end either a strap or a shackle (cf. U.S. Pat. Nos. 2,496,474 and 2,549,734), or a hook (cf. U.S. Pat. Nos. 2,916,300 and 3,863,955).
In the general case of such coupling gears incorporating a strap or a shackle, in most instances the pole is lifted manually. Even in the specific case of a mechanically-operated coupling gear comprising a hook lever, since the lifted load is relatively moderate, manufacturers are inclined to adopt a structure comprising a hook lever or relatively great length in order to take advantage of a relatively small raising angle. Manufacturers are also not expected to solve such problems as the consequence of the effort applied by the tractor to the pole and the coupling gear (this force being negligible), so that the position of the pivot axis of the lever on the support means fastened to the tractor, in relation to the rear axle thereof, can be choosen freely.
This is found notably in the above-mentioned U.S. Pat. Nos. 2,916,300 and 3,863,955, and even in the U.S. Pat. No. 2,549,734, in which the device is completely overhung, externally of the tractor.
In contrast thereto, in the case of a coupling gear for a semi-carried trailer, such as farming trailers without front prop or stand and wherein the pole is not pivoted to the frame, the lifting torque to be overcome by the lifting gear is relatively high (sometimes several tons).
This lifting and coupling gear requires a minimum distance between the trailer axle and the axis for pivoting the lifting lever to its support or bracket rigid with the tractor, in order to minimize the lever arm of the resistant moment.
Consequently, the lever must be extremely short, just as necessary to enable the hook portion thereof on the one hand to be accessible from the rear of the tractor and on the other hand to rest on the ground for picking up the shackle of the trailer.
Besides, the front pivotal mounting of this lever on the tractor, which constitutes the point of application thereto of the load carried by the trailer and transmitted by the pole, and also by the lever itself, should be located as close as possible to the rear axle of the tractor to prevent the latter from being reared by said load, or from impairing the adherence of the tractor front wheels to the ground. It may be noted that these various requirements are rather contradictory to one another, considering the data imposed by the original tractor structure, such as the importance of the ground clearance and of the rear overhanging portion in relation to the rear axle.